The present invention relates to gears chiefly for use in precision apparatus, such as timepieces and acoustic devices, and more particularly to gears which are operable with reduced meshing noises.
In recent years, gears of plastics have been widely used for timepieces, acoustic devices and like precision apparatus. When meshing, gears usually make noise (impactive noise) due to striking contact between the teeth. Gears in timepieces or the like which rotate intermittently and repeatedly specially produce impact noise every time the gears rotate. Gears of plastics, which are made by injection molding, involve eccentricity or some other strain, for example, due to shrinkage during molding and therefore make noise when meshing. Thus when gears involve eccentricity or errors in the pitch of the teeth, they fail to come into contact with each other at the specified portions of the opposing teeth, allowing for either interference of teeth or teeth meshing with each other with improper timing. Consequently, the meshing portions are subjected to marked variations of the load which give rise to vibration or noises. Such impact noises and noises due to strain or deformation are very disagreeable in the case of timepieces or acoustic devices.
To eliminate meshing noises by causing the gear to absorb impact and strain, it is already known to form in each tooth a cut groove which extends from the midportion of its ridge toward the axis to a given depth, or to form a cut groove in each furrow portion between the teeth so as to render the teeth elastically deformable by such grooves. However, in providing a gear of relatively small diameter, especially of a small module, with such a conventional structure in which the pitch of the teeth is small and therefore the cut grooves must have a very small width, extreme difficulties are encountered in making the gear when injection molding is used, because it is extremely difficult to make a mold which is shaped to form the grooves.
Further, since meshing noises are attributable to the torque acting on the teeth, it might appear useful to reduce the torque by increasing the diameter of the gear or the number of the teeth, but this is not feasible with timepieces and like precision devices in which it is necessary to use gears of small diameters.